1. Field of the Invention
The invention is related to the field of electric wireline well logging instruments for making measurements of the electrical resistivity of earth formations. More specifically, the invention is related to so-called "galvanic" electrical resistivity measuring instruments, which pass electrical current through various electrodes on the instrument and measure voltage differences between the electrodes to determine the formation resistivity.
2. Description of the Related Art
Electric wireline well logging instruments known in the art include galvanic resistivity instruments. Galvanic resistivity instruments make measurements related to the electrical resistivity of earth formations surrounding the wellbore in which the instrument is disposed. Typically galvanic resistivity instruments include a number of spaced apart electrodes disposed on an insulating part of the instrument. Some of the electrodes are used to emit electrical current into the wellbore, and then into the surrounding earth formations. Other ones of the electrodes are used to make measurements of voltage drop. Measurements of the voltage drop and of the magnitude of the electrical current which results in that voltage drop are used to determine the electrical resistivity of the earth formation.
Certain types of galvanic resistivity instruments are known as "unfocused" devices which include so-called "normal" and "lateral" measurements. See for example, E. L. Bigelow, "Introduction to Wireline Log Analysis", Western Atlas Logging Services, Houston, Tex. (1992) p. 57. Unfocused galvanic resistivity instruments typically inject current at one electrode and return it at another, making one or more different measurements of voltage drop and current magnitude at the same or other electrodes. The spacing between the electrodes used to inject the electrical current into the earth formations, and the spacing of the electrodes used to measure the voltage drop are related both to the vertical resolution of the resulting resistivity determinations and to the radial "depth of investigation" of the resistivity determinations. The latter term, depth of investigation, describes the radial distance from the central axis of the instrument where the formations for which the measurements are made are located. In general, as the electrode spacing increases, the vertical resolution of the measurements becomes more coarse (larger) while the radial depth of investigation becomes greater.
It is desirable to determine resistivity at a variety of radial depths within the earth formation particularly to determine whether and to what extent any fluid from the wellbore has displaced connate fluid in the pore spaces of the earth formation, and to determine the resistivity of the connate fluid in the earth formation where it has not been displaced. For this purpose some galvanic resistivity instruments include an array of injection and voltage drop measuring electrodes for determining the resistivity of the earth formations at a plurality of different radial depths of investigation within the earth formation. See for example, U.S. Pat. No. 2,920,266 issued to Owen and U.S. Pat. No. 3,697,864 issued to Runge.
Array galvanic resistivity devices such as shown in the Runge '864 patent and the Owen '266 patent have several limitations. First, the radial depth of investigation is limited, even at relatively long electrode spacings, particularly when the resistivity of the earth formations is substantially higher than the resistivity of the fluid in the wellbore. In such cases much more of the electrical current will flow within the wellbore fluid than will flow in the earth formations surrounding the wellbore. Second, some earth formations can be relatively deeply penetrated ("invaded") by the fluid from the wellbore. Measuring resistivity of the uninvaded formation using an array galvanic instrument would require such long electrode spacings as to make the instrument impracticably long. Further, the vertical resolution of the instrument at such long electrode spacings would be so coarse as to be unable to determine the formation resistivity within relatively thin earth formation layers.
A type of galvanic resistivity instrument known in the art as the "dual laterolog" instrument, can measure formation resistivity within relatively thin "layers" at relatively great radial depth of investigation even when the formation resistivity is much higher than the resistivity of the fluid in the wellbore. The principle of this instrument is described in the "Introduction to Wireline Log Analysis" reference on pages 58-59. The dual laterolog instrument includes a measuring current circuit and a focusing current circuit. The measuring current circuit passes electrical current from a source electrode through a the wellbore and the earth formations to a return electrode generally located at the earth's surface. The magnitude of the measuring current and its voltage drop are measured. The current magnitude and voltage drop are related to the resistivity of the earth formation. The layer within the earth formations for which the resistivity is measured is constrained by the focusing current. The focusing current is emitted by "guard" or "bucking" electrodes spaced symmetrically about the source electrode. The magnitude of the focusing current is continuously adjusted so that substantially no voltage drop occurs in a direction parallel to the axis of the instrument. The measuring current is therefore constrained to flow substantially radially outwardly from the instrument into the earth formations. The vertical resolution of the dual laterolog instrument is generally related to the axial spacing between the guard electrodes nearest to the measuring current source electrode.
The voltage drop of the measuring current, however, is related to the resistivity of every component along the path of the measuring current from the source electrode to the return electrode. These components include the fluid in the wellbore and the earth formations in which part or all of the connate fluids have been displaced. The measurements made by the dual laterolog instrument are therefore affected by the resistivity of the fluid in the wellbore, and the resulting resistivity of the earth formations in which the connate fluid has been partially or totally displaced by fluid from the wellbore. The dual laterolog instrument seeks to overcome this limitation by providing a second ("shallow") laterolog measurement which is intended to have a shallower radial depth of investigation. The shallow laterolog measurement is typically made by returning the focusing current to electrodes on the sonde mandrel rather than to the armor on the electrical cable used to convey the instrument, or to the earth's surface. This allows the measuring current to disperse at a relatively shallow radial depth within the earth formations.
The combination of "deep" and shallow laterolog measurements from the dual laterolog instrument has proven inadequate to resolve the radial distribution of resistivity in the earth formations proximal to the wellbore, because this resistivity distribution can vary to such as great degree depending on factors such as the hydraulic properties of the fluid in the wellbore, and the porosity and permeability of the earth formations. Both the deep and shallow laterolog measurements are affected by the distribution resistivity proximal to the wellbore. Further, in the case where the fluid in the wellbore is not very conductive as compared to the earth formations surrounding the wellbore, both the deep laterolog and the shallow laterolog measurements are subject to substantial error as a result of the relatively large amount of the total voltage drop which will occur in the formations proximal to the wellbore.
What is needed is a resistivity measuring instrument that can resolve the radial distribution of resistivity of earth formations while also providing relatively resistivity measurements having fine vertical resolution and great radial depth of investigation.